Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device using a variable resistive element that can store information by changing in electric resistance caused by applying of electric stress.
Description of Related Art
In recent years, the capacity of a nonvolatile semiconductor memory device as represented by a flash memory becomes remarkably larger. At the product level, products with a capacity of over 10 gigabytes are available at low prices. The value of the products is rising especially as portable or mobile memories such as USB memories, and the device is increasingly likely to grab shares of the market dominated by magnetic optical discs and the like. The capacity of several gigabytes is enough to serve as the storage of portable music players. Compared with a hard disk-equipped portable music player that has been rapidly becoming popular, a portable music player that is equipped with a nonvolatile semiconductor memory device, which is a solid-state element memory, has succeeded in appealing to users in terms of vibration resistance, high reliability, and low power consumption, which are, in principle, superior properties unique to the solid-state element memory. The solid-state element memory is expected to become mainstream as the storage of portable or mobile products for music and pictures.
Meanwhile, as candidates for next-generation nonvolatile semiconductor memory devices, which could overcome drawbacks of flash memories and are characterized by high-speed operation, research and development have been underway in recent years on nonvolatile memory elements that are each based on unique principles, such as a ferroelectric memory (FeRAM), magnetic memory (MRAM), phase change memory (PRAM), and resistance variable memory (RRAM) (Registered Trademark). As a method of rewriting information of the nonvolatile memory elements, there are two methods: a method of rewriting by using a bipolar voltage or current; and a method of rewriting by using a single polar voltage or current. FeRAM and MRAM employ the former driving method, while PRAM employs the latter. There are reports that RRAM are driven by both methods.
For example, RRAM is made up of a variable resistive element having a structure in which a variable resistor made of metal oxide is held between two electrodes. There are the following types: a type in which resistance is changed all over an electrode surface; and a filament type that has a local current path (called “filament”) between electrodes, and resistance is changed by opening and closing of the current path. A low-resistance state and a high-resistance state correspond to digital information “1” and “0,” respectively. In this manner, information can be recorded.
Prior to a resistance switching operation, a variable resistive element of a filament type is required to perform an initialization operation involving soft breakdown, which is called forming, in order to form a filament. As one example of RRAM, the technology disclosed in Baek, I. G. et al. “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses” IEDM2004, pp. 587-590, 2004 is known.
In RRAM having the filament-type variable resistive element, it is possible to control a current that flows after breakdown. However, in principle, it is difficult to control the period from when a voltage is applied until when the breakdown starts. FIG. 10 shows changes of current over time which flows at a time when a forming voltage pulse is applied at two variable resistive elements where hafnium oxide (HfOx), which functions as a variable resistor, is held between a Ta electrode and a TiN electrode. During an initial period of applying of a voltage pulse, the variable resistive elements are in a post-production initial high-resistance state. However, as the resistance rapidly drops due to the breakdown, the current starts to flow. As shown in FIG. 10, there is a difference between the elements in terms of the period from when a voltage pulse starts to be applied until when the breakdown starts. For example, in the case of FIG. 10, the period of the element 1 is short at about 70 ns, while the period of the element 2 is about 250 ns, which is longer than that of the element 1 (about 70 ns). However, given the nature of the breakdown, it is difficult to ensure that the breakdown start time of one element is exactly the same as that of the other element.
FIG. 11 shows one example of a forming process method in a memory cell array that includes a plurality of variable resistive elements. As for the forming in a memory array where a plurality of variable resistive elements are arranged, for example, as shown in FIG. 11, there is a method of sequentially applying a forming voltage pulse to each memory cell for the same period of time. One of gate voltages VW1 to VW3 of select transistors S1 to S3 is set to a High level, thereby selecting one of variable resistive elements R1 to R3. When a gate voltage V0 of a PMOS switch M0 is set to a Low level, VFMG is applied to a selected cell, and the forming is initiated. At this time, a forming current IFMG flows through the selected cell.
FIG. 12 shows a timing chart where the forming process is performed on R1, R2, and R3 in that order in the case of FIG. 11. Even if the same forming voltage pulse is applied, periods Δt1 to Δt3, which are the periods required for the breakdown to start, are different between the elements. After the breakdown of the variable resistive elements, the forming current continues to flow through the variable resistive elements until the applying of the forming voltage pulse is stopped. Therefore, periods u1 to u3, which are the periods during which the forming current flows after the breakdown, are different. The filament is formed by the breakdown, and a state in which resistance switching is possible is formed. However, if the periods u1 to u3, which are the periods during which the forming current flows, are different between the elements, a difference emerges between the elements in terms of the state of the filament. As a result, there is concern that a difference would occur between the elements in terms of resistance switching characteristics.
Therefore, what is desired is a nonvolatile semiconductor memory device in which variations of a forming current, which flows after breakdown, are reduced between elements.